Alternative Studied for Sensitizing TiO2 to Visible Light

To truly enable a transition to a global industrial system based on hydrogen rather than on fossil fuels will require the development of an efficient and environmentally friendly means of extracting hydrogen from feedstocks such as water. One proposed solution is to use a catalyst that would be driven by the energy of sunlight.

One of the best known and most successful photocatalysts is TiO2. The material does not respond to visible wavelengths, however, and the UV radiation to which it does respond comprises only a small percent of the solar energy falling onto the Earth.

A variety of means to sensitize TiO2 to visible light have been investigated, including the use of organic dyes, organometallic complexes and quantum dots, said Wonyong Choi of Pohang University of Science and Technology in South Korea. Choi and his research partner, Hyunwoong Park, have suggested an alternative method: coating TiO2 particles with Nafion, a cation exchange resin of interest for fuel cells and electrolyzers, and with an outer layer of the cationic ruthenium sensitizer Ru(bpy)2+3.

Particular advantages of their material, Choi said, are that it involves uncomplicated chemical synthesis methods and that it relies on electrostatic attraction to bind the sensitizer to the TiO2. Similar methods employ derivatized ligands that lead to the sensitized photocatalyst being insufficiently stable in water.

Although the material performed well in experimental tests, he said that its conversion efficiency — 2.6 percent in optimal conditions — is too low for commercial applications at this point. Moreover, as with all sensitized TiO2 photochemical methods, it is a sacrificial system and so requires additives such as organic acids and alcohol. And ruthenium sensitizers likely will be too expensive for mass applications.

The researchers are investigating whether Nafion-coated TiO2, without a bound sensitizer, can generate hydrogen from dye wastewater. Preliminary work suggests that the material degrades the dyestuff and that it produces hydrogen under visible light.

“We believe that we can accomplish both wastewater treatment and hydrogen production simultaneously, in a very environmentally friendly way,” Choi said.